The climate control system for the interior of many currently manufactured automotive vehicles comprises both a heater and an air conditioner. In an automotive vehicle having such a climate control system, both heater and air conditioner rely on a prime mover, such as an internal combustion engine, which propels the vehicle in order to provide heating and cooling of the vehicle's interior.
In an automotive vehicle which is propelled by an internal combustion engine, heating of the interior is provided by heat from engine coolant flowing through a heater core. A blower fan draws air through the heater core and forces heated air into the interior through a distribution system. Heating is controlled in any of various existing ways, either manually or automatically.
Manual control is by occupant adjustment of a temperature input device having a knob or lever for selecting temperature. If occupant-desired temperature is not maintained, the occupant must manually re-adjust the input device.
Automatic control of heating is accomplished by a controller using temperature feedback to modulate heating provided by the heater core for maintaining a desired temperature set by an occupant on a temperature selection device.
Cooling of the interior is provided by liquid refrigerant which evaporates as it passes through an evaporator in a refrigeration circuit having a mechanically operated compressor coupled through a clutch to the engine. With the engine running and the clutch engaged, the compressor draws vaporized refrigerant from the evaporator and pumps it through a condenser where heat of evaporation in the refrigerant is rejected to outside air causing the refrigerant to condense into liquid which is then pumped back through an expansion valve to the evaporator.
Because the compressor is being driven by the engine, the amount of cooling provided by the evaporator depends on engine speed. The compressor is sized and operated to deliver an amount of cooling which is considered sufficient for reasonable occupant comfort when the vehicle is stopped and the engine is running at idle speed. However, that significantly reduces overall compressor efficiency because when the vehicle is being propelled by the engine, the engine is operating the compressor at speeds which are significantly greater than speeds which provide best compressor operating efficiency. A typical Coefficient of Performance (COP) for automotive air conditioning systems is somewhere around 1.5. (COP is the ratio of cooling-delivered energy BTUs to input energy BTUs).
In certain commercial vehicles, an engine cooling fan is coupled through a clutch for operation by the engine to draw air through the condenser when the compressor is operating and internal refrigerant pressure exceeds a predetermined limit. At low vehicle speeds, cooling fan operation may create additional noise, as well as drawing dust and dirt through the condenser. Engine cooling fan operation contributes to inefficient air conditioning operation because typical fan operation uses significantly more power (perhaps as much as fifty or more horsepower) than that required to draw air through the condenser (typically less than one horsepower).
A thermostatic control is commonly used to control temperature when the interior is being cooled. When the temperature becomes greater than a selected temperature to which the thermostatic control is set, the compressor operates to cool air which is drawn by a blower fan across heat exchange surfaces of the evaporator and subsequently moved into the occupant compartment. When temperature becomes less than the selected temperature set by the thermostatic control, operation of the compressor is discontinued. A hysteresis margin of a few degrees may be provided between “compressor-on” and “compressor-off” to reduce the frequency of cycling the compressor back and forth between on and off. For the typical vehicle air conditioning system, the evaporator is operated in the 35°-45° F. (Fahrenheit) temperature range. An air mixing valve controls the ratio of air sourced from the evaporator with air sourced from the engine coolant heater core (heat exchanger) to achieve the selected temperature, with the driver adjusting recirculation fan speeds for comfort. It is noted that with the evaporator operating in the 35°-45° F. temperature range, the system is very effective for reducing relative humidity within the vehicle interior, even in a low relative humidity ambient environment.
A temperature sensor located at the condenser functions to disengage the compressor clutch if the sensed temperature is too low, thus avoiding build-up of ice on the condenser surfaces. Consequently, the compressor may cycle off and on as the system operates.
During cold weather operation, hot air from the heater core is mixed with cold air from the evaporator to produce warm and dry air for the defog/defrost function. Other mixing valves provide functions such as heating only, and venting with outside air.
In summary, the typical vehicle air conditioning system loses efficiency due to high compressor speeds, to the use of the engine coolant fan for condenser air flow, to the mixing of heated air from the heater core with cold air delivered from the evaporator, and to operation of the evaporator in the 35°-45° F. temperature range.